Abstract

Transverse plane rotation in an amputee's residual-limb appears to be less
than in non-amputees [1,2]. However, it remains unclear firstly, to what extent
this is driven by the properties of the prosthesis, and secondly, whether this
has detrimental effects on the residual-limb. In trans-tibial prostheses, the
socket and foot are commonly connected via a rigid adapter, which restricts
the residual-limb's transverse rotation while inside the socket and can increase
the forces exerted onto the residual-limb. This can subsequently trigger tissue
damage. Instead of a rigid connection, an adapter with a compliant element
can be fitted to prostheses to allow some transverse rotation. This novel study
quantified in-socket forces and amputated side kinetics and kinematics to
investigate the mechanisms by which the adapter reduces in-socket forces.
Gait tests were conducted with ten unilateral trans-tibial amputees who walked
on a prosthesis that had an adapter with (A) and without (B) a compliant
element. Comparing (A) with (B) showed that, at early stance, whilst the socket
rotated internally due to adapter compliance, the pelvis was therefore likely
to internally rotate and move forward more relative to the prosthesis. This
forward motion is associated with greater elevation of the centre of mass, which
can be compensated for by increasing and delaying the peak in knee flexion
during stance. Peak knee flexion magnitude was not significantly increased
with the adapter (approximately 0.6 °, p = 0.976). However, peak knee flexion
was significantly delayed with the adapter (approximately 3.28% of stance,
p=0.010). Knee flexion is a common shock absorption mechanism, which
significantly reduced the peak vertical ground reaction force (approximately
0.2N/kg, p=0.050) and delayed it, although not significantly (approximately
1.41% of stance, p=0.141), thus overall reducing the vertical loading rate.
Reduced in-socket forces supported these findings. In conclusion, adapter
compliance has therefore beneficial effects on residual-limb forces.